Carbazole end capped bipyridine compounds and process for preparation thereof

ABSTRACT

The present invention is to provide highly solid state emissive fluorophores of the formula 1 and 2 useful for the solid state lighting. The formula 1 and 2 have good solid state emission in blue and green region respectively. The zinc complexes of formula 1A and 2A have yellow and red emission respectively in solid state. The present invention also provide high solid state emissive assay of formula 1 and 2 to convert UV light into visible light by coating the color tunable materials on the 365-nm emitting solid state LED. The present invention further provides carbazole end-capped bipyridine for screening of metal salts with different counterions in solutions. The discrimination of the various metal salts can be monitored by noting the fluorescence change in solution. The zinc salts with different counterions have different emission color in solution. The present invention also develop a new two photon active fluorescent Zn2+ specific probe for detecting Zn2+ in cellular environments. Assay with formula 3 is used as a fluorescent probe for two photon imaging of Zn2+ ions in HeLa cells. The high two photon absorption cross section and fluorescence quantum yield of formula 3 made it as a useful probe for detecting Zn2+ in cellular environment by two photon excited fluorescence imaging.

FIELD OF THE INVENTION

The present invention relates to Carbazole end capped bipyridinecompounds of general formula A and their Zn— complex useful for solidstate lighting, screening of counteranions and two photon biologicalimaging.

The present invention also provides a process for the preparation ofgeneral formula A.

BACKGROUND OF THE INVENTION

The search for the development of tunable organic fluorophores withimproved properties are of great interest. Fluorophores with attractiveand efficient emission characteristics are desirable for applications inbiological science, material science application such as organic lightemitting diodes (OLEDs) and sensing applications. Reference may be madeto: a) E. M. Nolan, S. J. Lippard, Chem. Rev. 2008, 108, 3443; b)Applied Fluorescence in Chemistry, Biology, and Medicine (Eds.: W.Rettig, B. Strehmel, S. Schrader, H. Seifert), Springer, New York, 1999;c) A. C. Grimsdale, K. L. Chan, R. E. Martin, P. G. Jokisz, A. B.Holmes, Chem. Rev. 2009, 109, 897; d) S. M. Kelly in Flat PanelDisplays: Advanced Organic Materials (Ed.: J. A. Connor), The RoyalSociety of Chemistry, Cambridge, 2000; e) S. Park, J. E. Kwon, S. H.Kim, J. Seo, K. Chung, S.-Y. Park, D.-J. Jang, B. M. Medina, J.Gierschner, S. Y. Park, J. Am. Chem. Soc. 2009, 131, 14043; f) Y.Yamaguchi, Y. Matsubara, T. Ochi, T. Wakamiya, Z.-I. Yoshida, J. Am.Chem. Soc. 2008, 130, 113867; g) Z. M. Hudson, S. Wang, Acc. Che. Res.2009, 42, 1584; g) H. S. Joshi, R. Jamshidi, Y. Tor, Angew. Chem. Int.Ed. 1999, 38, 2722;

A critical element in designing and fabricating such materials is thecontrol of their emission wavelength. Blue-, green- andred-light-emitting materials are needed for full-color displays. Fororganic molecules this is often achieved by chemically modifying theπ-conjugation or the substituent group; which include substitution withstronger donor or acceptor moieties. This will effectively modulate theHOMO-LUMO gap of the molecules. An alternative approach for controllingthe emitted color of organic materials is to append fluorescentchromophores to a polymeric backbone or to blend such dyes into inertpolymeric matrices. Reference may be made to: a) Y. Yamaguchi, T.Tanaka, S. Kobayashi, T. Wakamiya, Y. Matsubara, Z.-I. Yoshida, J. Am.Chem. Soc. 2005, 127, 9332; b) R. Abbel, C. Grenier, M. J. Pounderoijen,J. W. Stouwdam, P. E. L. G. Leclere, R. P. Sijbesma, E. W. Meijer, A. P.H. J. Schenning, J. Am. Chem. Soc. 2009, 131, 833; c) G. Kwak, H. Kim,I.-K. Kang, S.-H. Kim, Macromolecules, 2009, 42, 1733.

Light emitting devices can be used in displays (eg., flat-paneldisplays), screens such as computer screens and other items that requireillumination. The brightness of the light emitting device is animportant feature of the device. Solid state light emitting devicesincluding LED's are extremely useful because of their low fabricationcosts and long term durability. Reference may be made to: Rubner et al.U.S. Pat. No. 6,548,836, Baretz et al. U.S. Pat. No. 6,600,175. As theorganic luminescent material, organic dyes emitting fluorescence such as8-quinolinol aluminium complex or coumarin compounds are used. Althoughthe organic light emitting diode has high luminescent characteristics,they involve a problem in the stability upon light emission or halflife. Reference may be made to: Hirose et al. U.S. Pat. No. 6,670,052

An organic light-emitting device in which an oxazole-, thiazole- orimidazole-fused phenanthroline molecule is used as an emissive layer byChen et al. U.S. Pat. No. 7,179,542, U.S. Pat. No. 6,713,781.

So far, a variety of strategies have been worked out to realize highsolid state emission. The modulation of optical-band gap by changing thestrength of donor-acceptor have been shown by Ajayaghosh and coworkers.Reference can be made to: a) A. Ajayaghosh, V. K. Praveen, S.Srinivasan, and R. Varghese, Adv. Mater. 2007, 19, 411; b) C.Vijayakumar, V. K. Praveen, and A. Ajayaghosh Adv. Mater. 2009, 21,2059, Ajayaghosh et al. PCT/IN2008/000372. Recently the emission of thethree primary colors (blue, green and red) simultaneously with equalintensities to produce white light and the pure colors separatly in atunable way was achieved from a single component emitter. Reference canbe made to: G. He, D. Guo, C. He, X. Zhang, X. Zhao, and C. Duan; Angew.Chem. Int. Ed. 2009, 48, 6132.

But the rational design of fluorescent probes with appreciable quantumyield in solid state is still a challenging task. Therefore the demandfor the design and development of efficient fluorescent materials arealways a matter of scientific concern. In the present invention we putforward a simple and easy protocol for developing organic luminescentmaterials with high solid-state emission useful for fabricatingmulticolor light emitting devices. The conversion of UV light intovisible light is also demonstrated by coating the color tunablematerials on the 365-nm emitting solid state LED.

Another application f fluorophores is for sensing of cations, anions andneutral molecules. Reference can be made to: a) A. P. de Silva, H. Q.Gunaratne, T. Gunnlaugsson, A. J. M. Huxely, C. P. McCoy, J. T.Rademacher, T. E. Rice, Chem. Rev. 1997, 97, 1515; b) M. Takeuchi, M.Ikeda, A. Sugasaki, S. Shinkai, Acc. Chem. Res. 2001, 34, 865; c) A.Ajayaghosh, Acc. Chem. Res. 2005, 38, 449; d) E. L. Que, D. W. Domaille,C. J. Chang, Chem. Rev. 2008, 108, 1517; e) E. Nolan, S. J. Lippard,Chem. Rev. 2008, 108, 3443; f) R. McRae, P. Bagchi, S. Sumalekshmy, C.J. Fahrni, Chem. Rev. 2009, 109, 4780; g) E. J. O'Neil, B. D. Smith,Chem. Soc. Rev. 2006, 250, 3068; h) S. W. Thomas III, G. D. Joly, T. M.Swager, Chem. Rev. 2007, 107, 1339.

Many of these sensors are specific either for a particular cation oranion. However, screening of a specific metal salts in terms of theassociated counter anions remains challenging. For example, colorimetricand/or fluorimetric probes that sense a specific cation will not ingeneral be able to differentiate the associated counteranions andvice-versa. Fluorophores with strong intramolecular charge transfer(ICT) shows substantial changes in fluorescence with respect to thesurrounding environment (solvatochromic probes). One of the methods toimpart solvatochromic property to a fluorophore is by functionalizationwith electron donor (D) and electron acceptor (A) moieties. Such astructure will cause significant variation in the dipole moment of themolecule between the ground state and excited state. If the donor andthe acceptor moieties are weak, charge transfer occurs in the excitedstate thereby perturbing the fluorescence. Fluorophores with positivesolvatochromism show red-shift in the emission maximum with low quantumyields on increasing the solvent polarity. In addition, solvatochromicprobes will also be associated with change in fluorescence lifetime withrespect to solvent polarity. Therefore, solvatochromic probes have beenwidely used for a variety of applications such as polarity sensitivelive cell imaging, cation sensing, and for biosensing. However,solvatochromic fluorescence property has not been exploited fordifferentiating and identifying the counter anions involved in differentsalts of a specific cation. In the present invention we show that thefluorescent molecular probe of formula 2 is able to distinguish zincsalts with various counterions. Reference may be made to: a) Sunahara,H.; Urano, Y.; Kojima, H.; Nagano, T J. Am. Chem. Soc. 2007, 129, 5597;b) S. Maruyama, K. Kikuchi, T. Hirano, Y. Urano, T. Nagano, J. Am. Chem.Soc. 2002, 124, 10650; c) S. Sumalekshmy, M. M. Henary, N. Siegel, P. V.Lawson, Y. Wu, K. Schmidt, J.-L. Bredas, J. W. Perry, C. J. Fahrni, J.Am. Chem. Soc. 2007, 129, 11888.

Certain fluorescence probes are highly fluorescing when excited with onephoton at a wavelength in UV or visible range. However such wavelengthsare inconvenient for cell imaging and tissue imaging because of theirlow penetration power and also due to the absorption of tissues andcells at this wavelength. Such wavelength also result in significantautofluorescence and phototoxicity. In order for the probe to be used asa two photon imaging probe for cells, it should be specific for aparticular analyte, have cell viability, and it should have good twophoton absorption cross section.

Many of the two photon absorbing compounds satisfy the formula D-π-D,A-π-A, D-A-D and A-D-A, wherein D is an electron donor group, A is anelectron acceptor group and π comprises a bridge of π-conjugated bondsconnecting the donor and acceptor. Molecules with these forms can bedesigned to operate in methods where in the compounds undergosimultaneous two photon absorption. Asymmetric dyes with large twophoton absorption cross section are prepared by Reinhardt et al.Reference may be made to: U.S. Pat. No. 5,770,737.

Novel two photon probe with high fluorescence quantum yield, high twophoton absorption cross section and high photostability are used in amethod of multi photon imaging. In that, the fluorophores arefunctionalized with moieties having the properties of covalentattachment onto proteins, antibodies, DNA and RNA. The two photon dyeswith high fluorescence intensity in the environment of cell membranesuseful for distinguishing hydrophilic and hydrophobic domains of thecell membranes are utilized for the real-time imaging of lipid rafts.Reference may be made to: Belfield et al. U.S. Pat. No. 7,253,287, Choet al. U.S. Pat. No. 7,511,167. Dipyrromethaneboron difluoride dyes andtheir conjugates used in bioanalytical assays that are based ontwo-photon excitation. Reference can be made to: Meltola et al. U.S.Pat. No. 7,763,439.

OBJECTIVES OF THE INVENTION

The main object of the present invention is to provide Carbazole endcapped bipyridine compounds of general formula A useful for solid statelighting and related applications.

Another object of the present invention is to provide a process for thepreparation of Carbazole end capped bipyridine compounds of generalformula A.

Another object of the present invention is to use the high solid stateemissive assay powder of general formula A to convert UV light intovisible light by coating the color tunable materials on the 365-nmemitting solid state LED.

Another object of the present invention is to provide a fluorophore offormula 2 for screening of metal salts with different counterions insolutions.

Yet another main objective of the present invention is to develop a newtwo photon active fluorescent Zn²⁺ specific probe for detecting Zn²⁺ incellular environments. The present invention aims to use Zn²⁺ specificfluorophore with formula 3 for two photon imaging of Zn²⁺ ions incellular environments.

SUMMARY OF THE INVENTION

Accordingly, the present invention relates to carbazole end cappedbipyridine compounds of general formula A and its zinc complex ofgeneral formula B

In one embodiment of the present invention the structural formulae ofthe representative compounds are:

4,4′-bis((E)-2-(9-decyl-9H-carbazol-3-yl)vinyl)-2,2′-bipyridine

5,5′-bis((E)-2-(9-decyl-9H-carbazol-3-yl)vinyl)-2,2′-bipyridine

Zinc complex of4,4′-bis((E)-2-(9-decyl-9H-carbazol-3-yl)vinyl)-2,2′-bipyridine (1A)

Zinc complex of5,5′-bis((E)-2-(9-decyl-9H-carbazol-3-yl)vinyl)-2,2′-bipyridine (2A)

In another embodiment of the present invention the compounds of generalformula A and its Zinc complex of general formula B are useful forconversion of UV light to visible light in solid state.

In another embodiment of the present invention the compounds 1 and 2 areuseful for conversion of UV light to blue color (490 nm) and green color(500 nm) in solid state respectively.

In another embodiment of the present invention the zinc complex 1A andthe zinc complex 2A convert UV light to yellowish orange light (586 nm)and red light (622 nm) respectively.

In another embodiment of the present invention the compound 2 is usefulfor the identification of counteranion in zinc salts in solution state.

In another embodiment of the present invention solvent used in solutionstate is chloroform.

In another embodiment of the present invention the counteranions of zincsalts is selected from the group consisting of chloride, acetate,bromide, perchlorate, nitrate and triflate.

In another embodiment of the present invention a process for thepreparation of compounds of general formula A and its Zinc complex ofgeneral formula B, wherein the said process comprising the steps;

-   -   a. adding substituted bipyridine and substituted carbazole        monoaldehyde in mole ratio 1:2 into a solvent in the presence of        a base;    -   b. stirring the reaction mixture as obtained in step (a) at        temperature ranging between 32-40° C. for a period ranging        between 12-14 h;    -   c. cooling the reaction mixture as obtained in step (b) to        obtain precipitate followed by dissolving precipitate in a        solvent and subsequently evaporating solvent to obtain crude        product;    -   d. purifying the crude product as obtained in step (c) by column        chromatography in basic alumina using petroleum ether as solvent        to obtain pure compounds of general formula A.    -   e. reacting the compound of general formula A with ZnCl₂ in mole        ratio 1:1 in a solvent preferably acetonitrile at room        temperature ranging between 30-35° C. to obtain the zinc complex        of formula B.

In another embodiment of the present invention solvent used in step (a)is selected from the group consisting of dry DMF or dry THF.

In another embodiment of the present invention base used in step (a) isselected from the group consisting of potassium tertiary butoxide andsodium hydride.

In another embodiment of the present invention substituted bipyridineused in step (a) is selected from group consisting of tetraethyl2,2′-bipyridine-5,5′-diylbis(methylene)diphosphonate and 4,4′-dimethyl-2,2′-bipyridine.

In another embodiment of the present invention substitute carbazolemonoaldehyde used in step (a) is 9-substituted carbazole-2-carbaldehyde.

In another embodiment of the present invention yield of Carbazole endcapped bipyridine compounds is in the range of 40-50%.

In another embodiment of the present invention a solid state emittingdevice prepared by using compound of general formula A and its zinccomplex of general formula B, wherein process steps comprising;

-   -   a) melting compounds of general formula A and its zinc complex        of general formula B on a mortar by heating up to 200-210° C.;    -   b) coating of said melt as obtained in step (a) over UV LED and        allowed to cool for 15-20 minutes to prepare a solid state        emitting device.

In another embodiment of the present invention solid state emittingdevice shows a solid state fluorescence quantum yield in the range of4.3% to 45.6%.

In another embodiment of the present invention Use of compound offormula 3 for two Photon Imaging of zinc ions in biological cellsselected from the group consisting of HeLa cell, Human muscle cells.

5,5′-bis((E)-2-(9-(2-(2-(2-methoxyethoxy)ethoxy)ethyl)-9H-carbazol-3-yl)vinyl)-2,2′-bipyridine(formula 3)

BRIEF DESCRIPTION ABOUT THE DRAWINGS

These and other features, aspects, and advantages of the presentinvention will become better understood with reference to the followingdescription and appended claims and accompanying drawings where:

FIG. 1: shows the emission spectral changes of a) 1 [6×10⁻⁶ M,λ_(ex)=358 nm] b) 2 [6×10⁻⁶ M, λ_(ex)=395 nm] in hexane (−), CHCl₃ (−),acetonitrile (−), DMSO (−).

Formula 1: shows the molecular structure of the ligand 1-3.

FIG. 2: shows emission spectral changes of 1 (−), 2 (−), 1•Zn²⁺ (−), and2•Zn²⁺ (−) in (Conc. 6×10⁻⁶ M in 1:1 CHCl₃/Acetonitrile, ex@ absorptionmaximum). b) Photographs showing the emission color changes.

FIG. 3: demonstrate the a) Solid state emission spectral changes of 1(−), 2 (−), 1•Zn²⁺ (−), and 2•Zn²⁺ (−). b) Photographs of correspondingemission color changes.

FIG. 4: shows the a) (1) A UV-LED (365 nm) illuminating blue light(commercially available from nitride semiconductors co., Ltd.), (2) LEDilluminating a coated layer of 1, (3) LED illuminating a coated layer of1•Zn² complex. b) (1) A UV-LED (365 nm) illuminating blue light (2) LEDilluminating a coated layer of 2, (3) LED illuminating a coated layer of2•Zn²⁺ complex.

FIG. 5: demonstrate the a) Emission spectral response of 2 (6×10⁻⁶ M) inchloroform upon addition of Zn(NO₃)₂ (0-1 eqv.). Inset figure shows theJob's plot showing the 1:1 binding of 2 to Zn(NO₃)₂. b) Plot offluorescence intensity of 2 (6 μM) monitored at 596 nm with differentmetal ions.

FIG. 6: Color change profile of formula 2 (6×10⁻⁶ M) upon addition ofmetal salts with different counteranions in chloroform (fluorescenceoutput from a BioTek cell reader, λex@ 435 nm).

FIG. 7: Fluorescence intensity versus wavelength based recognition ofmetals salts with formula 2.

FIG. 8: showing the a) one photon and b) two photon fluorescenceemission spectra for the titration of 3 [(a) Conc. 6×10⁻⁶ M (b) Conc.6×10⁻⁵ M in acetonitrile solution] with Zn(ClO₄)₂ in acetonitrile

FIG. 9: shows Two-photon action spectra of 3 (6×10⁻⁵ M in acetonitrilesolution) in the absence (▪) and presence (▴) of Zn²⁺.

FIG. 10: a-f shows the fluorescence image of C2C12 (Mouse myoblastcells) with 3 before (a-c) and after (d-f) Zn²⁺ incubation. Two photonmicroscopic images of HeLa cells with 3 before (g) and after (h) Zn²⁺incubation

FIG. 11. Scheme of synthesis of formula 1, 2 and 3

DETAILED DESCRIPTION OF THE INVENTION

The present invention is to provide highly solid state emissivefluorophores of formula 1 and 2 useful for the solid state lighting. Theformula 1 and 2 have good solid state emission in blue and green regionrespectively. The zinc complexes of formula 1 and 2 have yellow and redemission respectively in solid state. The present invention also providehigh solid state emissive assay of formula 1 and 2 to convert UV lightinto visible light by coating the color tunable materials on the 365-nmemitting solid state LED. The present invention further providescarbazole end-capped bipyridine for screening of metal salts withdifferent counterions in solutions. The discrimination of the variousmetal salts can be monitored by noting the fluorescence change insolution. The zinc salts with different counterions have differentemission color in solution. The present invention also develop a new twophoton active fluorescent Zn²⁺ specific probe for detecting Zn²⁺ incellular environments. Assay with formula 3 is used as a fluorescentprobe for two photon imaging of Zn²⁺ ions in HeLa cells. The high twophoton absorption cross section and fluorescence quantum yield offormula 3 made it as a useful probe for detecting Zn²⁺ in cellularenvironment by two photon excited fluorescence imaging.

Zinc complex of4,4′-bis((E)-2-(9-decyl-9H-carbazol-3-yl)vinyl)-2,2′-bipyridine

Zinc complex of5,5′-bis((E)-2-(9-decyl-9H-carbazol-3-yl)vinyl)-2,2′-bipyridine EXAMPLES

The following examples are given to illustrate the process of thepresent invention and should not be construed to limit the scope of thepresent invention.

Example-1 Preparation of Formula 1

2 mmol of 4 (4, 4′-dimethyl-2,2′-bipyridine) was dissolved in dry DMF(50 mL). Potassium tertiary butoxide (290 mg, 12 mmol). was added to thesolution and stirred well. 6 mmol of the carbazole monoaldehyde(9-substituted carbazole-2-carbaldehyde) (5) was added to the reactionmixture. Then the reaction was stirred at room temperature (32° C.) for12 h. After completion of the reaction the entire reaction mixture waspoured to ice. The yellow precipitate formed was dissolved in chloroform(100 mL) and concentrated. The crude product was purified by columnchromatography in basic alumina using petroleum ether as solvent. 1: mp195-197° C.; (Yield: 40%)

¹H NMR (300 MHz, CDCl₃, δ): 8.70 (d, 2H, Ar H), 8.60 (d, 2H, Ar H), 8.30(d, 2H, Ar H), 8.15-8.13 (d, 2H, Ar H), 7.74 (d, 1H, Ar H), 7.72 (s, 2H,Ar H), 7.67 (s, 1H, Ar H), 7.51-7.40 (m, 7H, Ar H), 7.29 (m, 2H, vinylicH), 7.21 (s, 2H, vinylic H), 7.16 (s, 1H, vinylic H), 4.33-4.29 (t, J=12Hz, 4H; —NCH₂), 1.89 (m, 4H; CH₂), 1.35-1.24 (m, 28H; CH₂), 0.89 (m, 6H;CH₃).

¹³C NMR (75 MHz, THF-d⁸, δ): 154.85, 147.33, 144.30, 139.13, 138.95,132.13, 123.77, 122.85, 121.37, 121.08, 118.38, 118.21, 117.52, 117.06,115.53, 107.07, 40.82, 29.97, 27.65, 27.09, 25.27, 20.69, 11.58; IR(KBr): v=3047, 2920, 2848, 1622, 1581, 1492, 1467, 1377, 1355, 1332,1238, 1143, 964, 866, 823, 752, 613; FAB-MS (m/z): [M+H]⁺ calcd forC₅₈H₆₇N₄, 819.53. found, 820.38.

Example-2 Preparation of Formula 2

A suspension of sodium hydride (12 mmol, 290 mg) in dry THF (50 mL) wasadded slowly to a solution of the bisphosphonate (tetraethyl2,2′-bipyridine-5,5′-diylbis(methylene)diphosphonate) (6) 912 mg) (2mmol) and the carbazole aldehyde (5, 9-substitutedcarbazole-2-carbaldehyde) (4 mmol, 1.35 g) in THF 50 mL). Afterrefluxing for 12 h, at 65° C. the reaction mixture obtained was cooledfollowed by the removal of the THF under reduced pressure to give ayellow solid residue. The residue was suspended in water 100 mL) andextracted with dichloromethane 50 mL). The organic layer was washed withbrine 50 mL) dried over Na₂SO₄ and concentrated to give the crudeproduct, which was further purified by column chromatography over basicalumina using petroleum ether as eluent.

2: mp 169-172° C.; Yield: 50%

¹H NMR (300 MHz, CDCl₃, δ): 8.82 (s, 2H, Ar H), 8.44-8.42 (d, 2H, Ar H),8.28 (s, 2H, Ar H), 8.17-8.15 (d, 2H, Ar H), 8.03-8.01 (d, 2H, Ar H),7.72-7.70 (d, 2H, Ar H), 7.52-7.40 (m, 8H, Ar H), 7.30-7.27 (d, 2H,vinylic, J=15 Hz), 7.19-7.16 (d, 2H, vinylic, J=15 Hz), 4.32-4.29 (t,4H—NCH₂, J=15 Hz), 1.90 (m, 4H; CH₂), 1.66 (s, 4H, CH₂), 1.40-1.37 (m,24H; CH₂), 0.90-0.88 (t, 6H; CH₃).

¹³C NMR (75 MHz, CDCl₃, δ): 154.20, 147.92, 140.89, 140.57, 133.52,132.93, 131.85, 127.91, 125.93, 124.56, 123.26, 122.84, 121.91, 120.85,120.47, 119.15, 119.06, 109.00, 43.24, 31.86, 29.40, 22.67, 14.12; IR(KBr): v=3051, 2956, 2922, 2850, 1624, 1597, 1469, 1381, 1330, 1259,1234, 960, 802, 744, 727; FAB-MS (m/z): [M+H]⁺ calcd for C₅₈H₆₇N₄,819.53. found, 820.59.

Example-3 Preparation of Formula 3

A suspension of sodium hydride (12 mmol, 290 mg) in dry THF (50 mL) wasadded slowly to a solution of the bisphosphonate 6, tetraethyl2,2′-bipyridine-5,5′-diylbis(methylene)diphosphonate, 912 mg, 2 mmol)and the carbazole aldehyde (5,9-(2-(2-(2-methoxyethoxy)ethoxy)ethyl)-9H-carbazole-3-carbaldehyde) (4mmol, 1.36 mg) in THF (50 mL). After refluxing for 12 h, at 65° C. thereaction mixture obtained was cooled followed by the removal of the THFunder reduced pressure to give a yellow solid residue. The residue wassuspended in water (100 mL) and extracted with dichloromethane (50 mL).The organic layer was washed with brine (50 mL) dried over Na₂SO₄ andconcentrated to give the crude product, which was further purified bycolumn chromatography over basic alumina using petroleum ether aseluent.

3: yield: 50%

¹H NMR (300 MHz, CDCl₃, TMS) δ (ppm): 8.81 (d, 2H, aromatic), 8.43 (d,2H, aromatic), 8.26 (s, 2H, aromatic), 8.14 (d, 2H, aromatic), 8.03 (dd,2H, aromatic), 7.72 (dd, 2H, aromatic), 7.47 (m, 8H, aromatic), 7.28(2H, vinylic), 7.20 (2H, vinylic), 4.52 (t, 4H—NCH₂), 3.89 (t, 4H,—OCH₂), 3.5 (m, 12H), 3.4 (m, 4H), 3.3 (s, 6H, —OCH₃).

¹³C NMR CDCl₃, (75 MHz) δ: 43.34, 59.00, 70.55, 71.01, 71.85, 109.17,118.95, 120.83, 122.19, 126.01, 128.19, 131.83, 132.98, 133.51, 140.80,141.05, 147.94, 154.24; FAB-MS: [M]⁺ Calcd for C₅₆H₆₂N₄O₈, 831.01. found832.66.

Example-4

The Zn complexes of 1 and 2 for the solid state lighting were preparesby mixing formula 1 and formula 2 with ZnCl₂ in 1:1 ratio with a littleacetonitrile (0.5 ml) at room temperature (35° C.). The solid powder ofthe assays was obtained by evaporating the solvent under reducedpressure. The color of the zinc complex of assay 1 was yellow and thatof assay 2 was orange in color. The powders were highly fluorescentunder UV light (365 nm). The emission color of zinc complex of assay 1was yellowish-orange and assay 2 was red under 365 nm UV light. Thepowder of the assays was used for coating on UV-LED.

Zinc complex of4,4′-bis((E)-2-(9-decyl-9H-carbazol-3-yl)vinyl)-2,2′-bipyridine (Formula1A)

Zinc complex of5,5′-bis((E)-2-(9-decyl-9H-carbazol-3-yl)vinyl)-2,2′-bipyridine.(Formula 2A) Example-5

Formula 1 shows an absorption maximum at 377 nm in chloroform. It showsan intense blue emission in hexane (419 nm) when excited at 360 nm.Which was further shifted to 467 nm in chloroform and to 472 nm inacetonitrile and DMSO with an enhancement in intensity. The increase inthe quantum yield in acetonitrile (ϕ_(f)=0.109) when compared to that inchloroform (ϕ_(f)=0.062) is due to the decrease in nonradiative decaypath way in acetonitrile. The 53 nm bathochromic shift shown by themolecule from nonpolar to polar solvents is due to the emission from thesolvent relaxed state. Fluorescence decay profiles shows an increase inlifetime from hexane to acetonitrile with a monoexponential decay.Formula 2 showed a broad and red-shifted absorption band at 405 nm inchloroform, due to the increase in conjugation. Formula 2 shows agradual and large solvatochromism in nonpolar to polar solvents. Formula2 shows an emission maximum at 430 nm in hexane when excited at 395 nmwhich was further shifted to 513 nm in DMSO. This is a clear indicationof excited state intraligand charge transfer. The monoexponential decayexcited state lifetime of 2 gradually increase from hexane (0.81 ns) toDMSO (1.59 ns).

The formula 1 and 2 are structurally similar except the linking positionof donor to the acceptor. In formula 1 the donor group (carbazole) isconnected to bipyridine at the 4,4′-position, but in the case of formula2 it is connected to the 5,5′-position. In both the cases the acceptoris a good metal chelator, hence binding of metal ions can also makesignificant changes in the photophysical properties of formula 1 and 2.The absorption band of formula 1 was shifted to 416 nm upon binding withZn(ClO₄)₂. Corresponding changes was also observed in emissionproperties. The emission maximum was shifted to 563 nm from 468 nm (blueto yellow) (FIG. 2). The binding decreased the nonradiative decay pathway of the excited state. The solution state quantum yield of the zinccomplex of formula 1 was found to be 0.49 (rhodamine B as the standard)which was initially showing a quantum yield of 0.109 in acetonitrile. Inthe case of formula 2 the absorption was shifted to 452 nm and emissionto 615 nm from 507 nm (green to orange) (FIG. 2) upon complexation withZn(ClO₄)₂. Assay 2 has a quantum yield of 0.62 in acetonitrile. Thecomplex shows a fairly good quantum yield of 0.304 in solution. Both thecases shows a 1:1 metal-ligand complex.

The red-shift in the absorption and emission wavelength of formula 2 isdue to the more effective conjugation in formula 2 when compared toformula 1. The complexation of 1 and 2 with d¹⁰ metal ions like Zn²⁺will enhance the intraligand charge transfer (ILCT). This will furthershift the absorption and emission to higher wavelength, when compared tothe free ligands formula 1 and 2.

Example-6

Solid state lighting needs the chromophore which emits in the solidstate. Most of the molecules shows self quenching of fluorescence insolid state. This problem preclude these molecules from a practicalapplication. The molecules described here are highly solid stateemitting. The molecule with high solid state emission was utilized forthe detection of zinc ions in aqueous condition. Reference can be madeto: Ajayaghosh et al. PCTIN2008000374, S. Sreejith, K. P. Divya and A.Ajayaghosh, Chem. Commun., 2008, 2903. Here the carbazole appendedfluorophores show fairly good quantum yield in the solid state which wasmeasured by absolute method (integrated sphere method). The formula 1, 2and their Zinc complexes showed slight red shift in their emissionmaximum. (FIG. 3) in the solid state. The formula 1 has blue emissionwith a quantum yield of 0.149 and its Zn²⁺ has yellow emission withquantum yield of 0.086. Formula 2 has green emission with quantum yieldof 0.456 and the Zn²⁺ complex of formula 2 has red emission with aquantum yield of 0.043.

The assay 1 The photophysical parameters in solid state as well as thesolution state are given in table 1.

Example-7

FIG. 4 shows emission in the entire visible range (blue to red) from thefluorophore coated LED's. We purchased commercially available UV-LED's(365 nm, 20 mA) from Nitride semiconductors co., Ltd. The UV-LED whencoated with compound 1, 2 and their metal complexes emit light in theentire visible region. Formula 1 and 2 are melted by heating in a mortarat 200° C. The solid melt was coated over UV LED and allowed to cool for20 minutes. Now the LED is ready for lighting. The melts of zinc boundformula 1 and 2 are also coated on the UV LED in the same way.

Example-8

The emission spectrum of formula 2 showed large dependency on solventpolarity. In hexane a structured emission spectrum was obtained withemission maximum at 433 nm (C=6×10⁻⁶ M, λ_(ex)=400 nm). In chloroform,the emission spectrum showed a maximum at 476 nm and become broader andshifted to longer wavelength region in acetonitrile (λ_(em)=505 nm) andDMSO (λ_(em)=515 nm) with an emission color change from blue to green(FIG. 1b ). This large solvatochromism in emission shown by formula 2could be due to the stabilization of the charge transfer state in theexcited state. Since the excited state is polar in nature, will be morestabilized in polar solvents. This is further proved by the timeresolved fluorescence lifetime studies. The molecule is highly emissivein less polar solvent like chloroform and showed a quantum yield of 87%(quinine sulphate in 0.1 N H₂SO₄ as standard) while emission intensitydecreased in acetonitrile (ϕ_(f)=69%).

Addition of transition metal salts to a solution of formula 2 (6×10⁻⁶ M)in chloroform showed a significant decrease in the absorption band at405 nm with the concomitant formation of a new red-shifted band ataround 460 nm. The new band indicate the planarization of the metalbound fluorophore and also the decrease in the HOMO-LUMO gap due to thestabilization of the LUMO on metal binding. The emission of formula 2 at476 nm was significantly quenched by different transition metal ionsexcept zinc (II) and cadmium (II). In the case of Zn²⁺ the emission bandat 476 nm was quenched with a concomitant formation of a new red-shiftedband. For example, the fluorescence response of formula 2 in chloroformagainst Zn(NO₃)₂ is shown in FIG. 5a . Upon titration of Zn(NO₃)₂ Theemission maximum is gradually decreased with concomitant formation of anew band at 563 nm through an isoemissive point at 547 nm. Theindividual emission response of formula 2 against different transitionmetal ions is shown in FIG. 5b . The selectivity was checked byrecording the fluorescence of formula 2 in presence of 5 times excess ofdifferent cations. Surprisingly we found that the red-shifted emissionof 2+Zn²⁺ was strongly dependent upon the counteranion of the zinc salt.The individual emission response of formula 2 with various zinc saltsshowed a remarkable change (FIG. 6). This observation further extendedour investigation to find out the ability of formula 2 to discriminatezinc (II) ions with various counteranions. The emission peak of formula2 in chloroform (λ_(em), =476 nm) was red-shifted to 597 nm on bindingwith Zn(ClO₄)₂ with a quantum yield of 0.30. The emission peak was blueshifted in the case of Zn(OAc)₂ (λ_(em)=548 nm, ϕ_(f)=0.50) incomparison with ZnCl₂ (λ_(em)=554 nm, ϕ_(f)=0.50) and Zn(NO₃)₂(λ_(em)=563 nm, ϕ_(f)=0.40). The large shift in the emission maximumdepending on the counteranion is due to the different coordinatingability between the counteranion and zinc (II). The net chargequantities of zinc cations depend on the average distance between zinccations and counteranion, which is determined by the ionizationcoefficient of the zinc salt in solution. Zinc (II) may possess more netcharge in the case of ClO₄ ⁻ as the counteranion due to the largerionization equilibrium coefficient of Zn(ClO₄)₂ in solution. Because ofthe more covalent bond property of Zn(OAc)₂, the net effective charge onzinc (II) will be low. The Job plot revealed a 1:1 complexation betweenthe fluorophore and the Zn²⁺ in all cases. The binding constantscalculated from Benesi-Hildebrand plots shows that the binding constantis highest for Zn(ClO₄)₂ (3.33×10⁵ M⁻¹) and it decreases from Zn(NO₃)₂(1.47×10⁵ M⁻¹) to ZnCl₂ (7.8×10⁴ M⁻¹); Zn(OAc)₂ shows the lowest value(1.77×10⁴ M⁻¹) in chloroform. The possibility to monitor two emissionmaxima with a single wavelength excitation makes formula 2 as anefficient ratiometric probe for various zinc (II) salts (FIG. 7). Thevariation in the fluorescence properties is not observed in the case ofnon-binding metal salts with different counterions. For example, we havechecked the effect of various sodium salts like perchlorate, nitrate,chloride and acetate. But shows no variation in the fluorescenceproperty of formula 2.

The detailed photophysical properties of the formula 2 on complexingwith various zinc salts are summarized in table 2. The stabilization ofthe excited state is maximum in the case of Zn(ClO₄)₂ complex. Itgradually decreases as the ionization of the zinc complex decreases.This is clear from the quantum yield and lifetime values of 2•Zn²⁺complexes. The effective discrimination of various zinc salts isaccomplishable by using the probe formula 2 since the excited state offormula 2 is CT state. So that the fluorescence can be easily perturbedby the stabilization of excited state.

Example-9

Screening of the counteranions can be done by recording the emissioncolor of formula 2 in the presence of different metal salts using aBioTeck cell reader upon excitation at 435 nm (FIG. 6). Controlexperiment in the absence of any metal salts showed the original bluefluorescence of the probe formula 2. The plot of fluorescence intensityversus wavelength can apply for the determination of metal salts whichare not easy to find out by visual color change (FIG. 7). The changes inemission properties are highly solvent dependent. For example, theemission shift of formula 2 on binding with various zinc salts inacetonitrile is negligible. Since the emission of formula 2 arises fromthe stabilized charge transfer state, the counteranion effect isnegligible in acetonitrile. The quantum yield of 2•Zn²⁺ complex wasnearly the same in acetonitrile (ϕ_(f)=0.30, rhodamine B in ethanol asstandard) irrespective of the counteranion whereas in chloroform thequantum yield was dependent on the counteranions (Table 2).

Example-10

Formula 3 has an absorption maximum at 406 nm in acetonitrile. It showsan emission maximum at 510 nm in acetonitrile upon excitation at 390 nm.The emission maximum was shifted to 525 nm in 1:1 acetonitrile/water,without significant shift in the absorption maximum. The emissionmaximum was red-shifted to 611 nm upon addition of 1 equivalent of ZnCl₂to solution of formula 3 in acetonitrile or 1:1 acetonitrile/water. Theemission intensity of zinc complex of formula 3 was reduced uponexcitation at 440 nm compared to free formula 3.

Formula 3 was showing two photon absorption (TPA) cross section value of107 GM at 800 nm. Interestingly the Zn²⁺ complex of formula 3 shows a 12fold enhancement in the TPA cross section in acetonitrile solution. Twophoton excitation at 860 nm produced ratiometric emission spectra with ared-shifted emission maximum for formula 3 on binding with Zn²⁺. The twophoton excited fluorescence spectra shows an enhancement in the emissionintensity for 3•Zn²⁺ complex when comparing to free formula 3. This isin contrast to the steady state fluorescence behaviour, wherecoordination with Zn²⁺ decreases the fluorescence intensity. Thisindicates that coordination with Zn²⁺ result in significant enhancementin the two photon excited fluorescent (TPEF) signal along with theincrease in the TPA cross section.

The two photon excitation was carried out using a mode lockedfemtosecond Ti: sapphire laser from Spectra Physics (Tsunami), with arepetition rate of 82 MHz and a pulse width of about 100 femtoseconds.The tuning range of the Ti-Sapphire laser is 690 nm to 1080 nm. Theoutput laser beam, with an average power of 0.6-0.8 W, is verticallypolarized.

Example-11

The two photon microscopic images of HeLa cells labeled with formula 3showed bright TPEF. The cells incubated with Zn²⁺ when imaged withformula 3 showed an enhanced TPEF at 595-640 nm. So formula 3 can beused as an efficient fluorescent molecular probe for selective detectionof intracellular free Zn²⁺ ions in living cells by two photonmicroscopic imaging.

The Main Advantages of the Present Invention are:

-   -   1) A clever design of donor-acceptor-donor based fluorophore and        their positional modification and metal ion binding assisted the        solid state lighting in the entire visible region. The        combination of change in linking position and metal complexation        helped to attain full color tenability in the visible region.        Here, we have successfully demonstrated the conversion of UV        light into visible light by coating the fluorophores on 365-nm        emitting LED. Thus with a blue emitting LED we could obtain the        entire visible spectrum such as blue, green, orange and red.    -   2) The excited state charge transfer property of the fluorescent        probe formula 2 is successfully utilized for the effective        screening of transition metal salts with different        counteranions. The easy synthesis, high sensitivity and        selectivity of the present probe make it a potential candidate        for the discrimination of transition metal salts. The present        probe can be employed for the selective visual sensing of zinc        salts with various counterions in analytical samples.    -   3) Formula 3 can be used as an efficient ratiometric fluorescent        molecular probe for selective detection of intracellular free        Zn²⁺ ions in living cells by two photon microscopic imaging.

The invention claimed is:
 1. A zinc complex of carbazole end cappedbipyridine compound of formula B, wherein the compound converts UV lightto visible light in solid state.


2. The carbazole end capped bipyridine compound as claimed in claim 1,wherein the structural formulae of the representative compound are:

4,4′-bis((E)-2-(9-decyl-9H-carbazol-3-yl)vinyl)-2,2′-bipyridine,

5,5′-bis((E)-2-(9-decyl-9H-carbazol-3-yl)vinyl)2,2′-bipyridine,

Zinc complex of4,4′-bis((E)-2-(9-decyl-9H-carbazol-3-yl)vinyl)-2,2′-bipyridine, (1A)

Zinc complex of5,5′-bis((E)-2-(9-decyl-9H-carbazol-3-yl)vinyl)-2,2′-bipyridine (2A). 3.A zinc complex of carbazole end capped bipyridine compound of formula B:


4. The zinc complex of carbazole end capped bipyridine compound asclaimed in claim 3, wherein the structural formulae of therepresentative compound are:

Zinc complex of4,4′-bis((E)-2-(9-decyl-9H-carbazol-3-yl)vinyl)-2,2-bipyridine (1A)

Zinc complex of5,5′-bis((E)-2-(9-decyl-9H-carbazol-3-yl)vinyl)-2,2′-bipyridine (2A).